User Guide
Sound System Design Reference Manual
Adding a 6 dB safety factor gives us the usual
form of the equation:
Maximum gain =
20 log D
0
- 20 log D
s
+ 20 log D
1
- 20 log D
2
- 6
In this form, the gain equation tells us several
things, some of them intuitively obvious:
1. That gain is independent of the level of the
talker
2. That decreasing
D
s
will increase gain
3. That increasing
D
1
will increase gain.
The Influence of Directional Microphones
and Loudspeakers on System Maximum
Gain
Let us rework the example of Figure 4-3, this
time making use of a directional loudspeaker whose
midband polar characteristics are as shown in Figure
4-4A. It is obvious from looking at Figure 4-4A that
sound arriving at the microphone along the
D
1
direction will be reduced 6 dB relative to the
omnidirectional loudspeaker. This 6 dB results
directly in added gain potential for the system.
The same holds for directional microphones, as
shown in Figure 4-5A. In Figure 4-5B, we show a
system using an omnidirectional loudspeaker and a
cardioid microphone with its -6 dB axis facing toward
the loudspeaker. This system is equivalent to the one
shown in Figure 4-4B; both exhibit a 6 dB increase in
maximum gain over the earlier case where both
microphone and loudspeaker were omnidirectional.
Finally, we can use both directional
loudspeakers and microphones to pick up additional
gain. We simply calculate the maximum gain using
omnidirectional elements, and then add to that value
the off-axis pattern advantage in dB for both
loudspeaker and microphone. As a practical matter,
however, it is not wise to rely too heavily on
directional microphones and loudspeakers to make a
significant increase in system gain. Most designers
are content to realize no more than 4-to-6 dB overall
added gain from the use of directional elements. The
reason for this is that microphones and loudspeaker
directional patterns are not constant with frequency.
Most directional loudspeakers will, at low
frequencies, appear to be nearly omnidirectional. If
more gain is called for, the most straightforward way
to get it is to reduce
D
s
or increase
D
1
.
Figure 4-4. System gain calculations,
directional loudspeaker
Figure 4-5. System gain calculations,
directional microphone
4-3